It is known that the flight distance of golf ball is greatly influenced by the relationship between the primary natural frequency (BF.sub.1) of a golf ball and the primary natural frequency (KF.sub.1) of a club head. In general, the closer the primary natural frequency (BF.sub.1) of a ball and the primary natural frequency (KF.sub.1) of a club head are to each other, the better the matching of the mechanical impedance therebetween becomes when the golf ball is hit with the golf club. This produces large impact resilience, which results in a long flight distance. Commercially available golf balls have a primary natural frequency of about 600 to 1600 Hz. Golf clubs with a club head made of persimmon, which are typical wood-type golf clubs, have a primary natural frequency of about 1800 to 2800 Hz. In order to produce a longer flight distance, one considers reducing the primary natural frequency of a golf club or increasing the primary natural frequency of a golf ball. The term "primary natural frequency" indicates a frequency measured when the mechanical impedance takes a primary minimum value.
Recently, golf clubs with a head made of stainless steel and titanium alloy, which produce long flight distance, have been mainly used as wood-type golf clubs. The golf club with a stainless steel head has a primary natural frequency (KF.sub.1) of about 1800 to 2500 Hz, and the golf club with a titanium alloy head has a primary natural frequency (KF.sub.1) of about 1400 to 1600 Hz. Both of these values are smaller than the primary natural frequency (KF.sub.1) of golf clubs with a head made of persimmon. The primary natural frequency (KF.sub.1) of the golf club is proportional to the spring constant thereof. Therefore, when the spring constant of the club head is lowered, the primary natural frequency (KF.sub.1) thereof is also lowered. As methods for reducing the spring constant of the club head, one conceives using a club head having a face with a thin thickness, or using a club head made of a material having small modulus of elasticity. However, such methods generally lower the strength and the hardness of the club head, and as a result, the durability and the resistance to flaw of the club head are deteriorated. Under such a situation, a limitation on reducing the primary natural frequency (KF.sub.1) of the club head to a value close to the primary natural frequency (BF.sub.1) of the golf ball is present. At present, the titanium alloy club head is considered to have the lowest possible primary natural frequency (KF.sub.1).
Due to such a problem, in an actual operation, the primary natural frequency (BF.sub.1) of a golf ball is increased so as to be close to that of the titanium alloy club head. However, when the primary natural frequency (BF.sub.1) of a golf ball is increased, its hardness is also increased. In this case, the golf ball produces a long flight distance and, the impact when hitting the ball becomes larger. It has been conventionally said that, although the commercially available golf balls having the primary natural frequency (BF.sub.1) of 1000 Hz or higher produce a long flight distance, they give the golf players the large impact when hit (i.e., they give golf players the feeling of hitting a hard golf ball).
In addition, it is also important to keep a good hit sound hit. A low hit sound gives the golf player an impression that the flight distance is short, regardless of whether or not the actual flight distance is long.